I. Field of the Invention
The present invention relates to a predictive coding apparatus of a television signal. More particularly, the present invention relates to an adaptive predictive coding system wherein a differential signal (i.e., prediction error signal) between a signal to be transmitted and a prediction signal is encoded to decrease the number of bits of data to be transmitted, and an optimum prediction function among a plurality of predictive functions is selected to provide a minimum data quantity for transmission.
II. Description of the Prior Art
Conventionally, in an interframe coding system, a prediction error signal having a large amplitude does not often occur in a still image or a quasi-still image with little motion, so that a total amount of data is small. However, the total amount of data is increased in a moving picture. A picture having little motion can be effectively transmitted by a small amount of data, whereas a moving picture must be transmitted with a great amount of data. An attempt has been made to improve transmission efficiency of a moving picture. For example, movements of a picture which are indicated by television signals are often regarded as translational displacements. Therefore, an interframe prediction (motion-compensated interframe prediction) method is proposed to compensate for the motion in accordance with the translational displacement of an object on the screen. This system is the most effective method of achieving a high coding efficiency for a moving picture.
In the motion-compensated interframe prediction system of this type, a plurality of prediction functions are used to keep coding efficiency high with respect to various types of motion. The plurality of prediction functions, which correspond to motion at various speeds and in various directions, are prepared, and the most suitable prediction function for an actual motion is defined as an optimum prediction function. In general, the optimum prediction function minimizes a prediction error. In this sense, the optimum prediction function is referred to as a motion vector hereinafter. Therefore, the motion-compensated interframe prediction system can be regarded as an optimum predictive coding system for determining the optimum prediction function (motion vector) among a plurality of prediction functions.
Conventionally, a considerably high transmission rate is adopted, so that motion vector data occupies only a slight portion of data to be transmitted and can be neglected. It has been considered that optimum prediction can be performed to decrease a total amount of transmission data by detecting the motion vector for minimizing the prediction error data amount. However, when the transmission rate is decreased, the motion vector data occupies a relatively large portion of the data to be transmitted and can no longer be neglected.
Furthermore, in the conventional practical coding apparatus, various types of coding or digitizing operations are performed to suppress rapid change of the amount of data generated even if the picture contents of the input television signals change. For example, a television signal to be coded is subsampled or subline coded. A plurality of quantization characteristics which comprise either a great number of quantization levels or a small number thereof are adaptively selected in accordance with the picture contents. In this case, the relationship between the prediction errors occurring in respective coding control modes and required amounts of data varies in the respective coding control modes. As a result, the proper amount of data cannot be calculated, thus impairing detection of correct motion vector.